Pressure vessels for storing compressed gases are of course very well known and are commonly referred to as “gas cylinders” because they conventionally, but not necessarily, have a cylindrical shape. A gas cylinder is typically able to store gas at a pressure of up to 200 bar or 300 bar. A gas cylinder is conventionally formed as a one-piece vessel, symmetrical about a longitudinal axis, from a suitable steel. The walls of the vessel have a suitable thickness to be able to withstand cycling of pressure from atmospheric pressure to a maximum storage pressure, which as stated above may be as high as 300 bar. The cylinder has an axial opening in which is fitted a valve, typically having a brass body with external screw threads that engage complementary screw threads in the mouth of the cylinder. There are various different kinds of cylinder valve, but all kinds are able to be opened to permit the controlled discharge of gas from the cylinder and also, when the cylinder is empty, to enable it to be refilled. Some cylinder valves have an integral pressure regulator to enable the user to set the pressure at which gas is delivered. Alternatively the cylinder valve may have a port in which a separate pressure regulator may be docked. The pressure regulator may include a pressure gauge to indicate the delivery pressure to a user. Such a pressure gauge is not in continuous communication with the interior of the cylinder.
The traditional steel gas cylinder is heavy and can be awkward to handle. Lighter weight alternatives are known and are increasingly being used, particularly for the storage of medical gases. A lightweight gas cylinder comprises an inner vessel, sometimes known as a liner, which is reinforced with suitable fibrous material, for example, carbon fibre, and which is encased in an outer plastics shell. The inner vessel is typically made of steel, but because of the fibre reinforcement, has a smaller wall thickness than the traditional steel gas cylinder. The inner vessel is typically formed in two pieces which are welded together.
It is a feature of both the traditional steel gas cylinder and its lightweight alternative, that when not in use, there is no simple way of telling how full or how empty it is, the mass of the gas in even a full cylinder being small or negligible in comparison with the mass of the cylinder itself.